1. Field of the Invention
The present invention relates to an optical-signal transmitting apparatus, an optical-signal receiving apparatus and an optical-signal transmitting and receiving system. More particularly, the invention relates to an optical-signal transmitting apparatus that scatters signal light to perform transmission, and an optical-signal receiving apparatus that receives the scattered light from the transmitting apparatus. The invention also relates to an optical-signal transmitting and receiving system that scatters signal light to transmit it and receives the scattered light.
2. Description of the Related Art
Hitherto, a light-emitting diode or a semiconductor laser having an oscillation wavelength of approximately 0.8 .mu.m is usually employed as a light source for use in a Local Area Network (LAN) using optical space transmission (hereinafter simply referred to as "optical radio transmission") for indoor use. However, light sources of the above type present the following problems.
An explanation will first be given of the problems encountered when a light-emitting diode is used as a light source. The modulation frequency of the diode does not fall under a sufficiently high frequency band as is required for performing full-duplex transmission of moving pictures demanded by a multimedia environment. This imposes a restriction on the range of uses of the LAN using optical radio transmission. Moreover, a light-emitting diode which can modulate with a high frequency, for examples 30 MHz, produces an optical output as low as 10 mW whereby sufficient optical power cannot be obtained. It is thus necessary to use a plurality of light-emitting diodes in order to increase the optical output, which accordingly requires the formation of a larger signal transmitting apparatus.
On the other hand, when a semiconductor laser is used as a light source, a laser having an oscillation wavelength of approximately 0.8 .mu.m is usually employed as a light source because of its availability. Because of a concern for the safety of the eyes, as will be described below, a semiconductor laser has a limited maximum permissible exposure (MPE) and thus cannot exhibit sufficient optical power in an oscillation wavelength of approximately 0.8 .mu.m for its application to the LAN using optical radio transmission. This problem is particularly noticeable when a scattered light signal is used.
Additionally, a wavelength of approximately 0.8 .mu.m of a semiconductor laser is in proximity to the major wavelength band of external waves of light emitted from a fluorescent lamp, an incandescent lamp or sunlight incident indoors, thereby easily causing leakage of noise from the above lights. It is thus necessary to provide a narrow-band filter (interference filter) which allows signals having a wavelength strictly coinciding with the wavelength of a semiconductor laser to pass therethrough in order to increase the signal to noise (S/N) ratio of a receiving signal. This incurs an increase in the costs.